Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

We introduce density functional theory and review recent progress in its application to transition metal chemistry. Topics covered include local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and catalysis, including molecules, clusters, nanoparticles, surfaces, and solids.

Density functional theory for transition metals and transition metal chemistry

The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Intrinsic ripples in graphene

and Applications of Fullerenes, Carbon Dots, Nanotubes, Graphene, Nanodiamonds, and Combined Superstructures Vasilios Georgakilas,† Jason A. Perman,‡ Jiri Tucek,‡ and Radek Zboril*,‡ †Material Science Department, University of Patras, 26504 Rio Patras, Greece ‡Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University in Olomouc, 17 listopadu 1192/12, 771 46 Olomouc, Czech Republic

Broad family of carbon nanoallotropes: classification, chemistry, and applications of fullerenes, carbon dots, nanotubes, graphene, nanodiamonds, and combined superstructures.

Large-Scale Fabrication of Boron Nitride Nanosheets and Their Utilization in Polymeric Composites with Improved Thermal and Mechanical Properties

The electronic and magnetic properties of single 3d transition-metal (TM) atom (V, Cr, Mn, Fe, Co, and Ni) adsorbed graphdiyne (GDY) and graphyne (GY) are systematically studied using density functional theory (DFT). It is found that the electronic structures of TM-GDY/GY are sensitive to the value of the on-site Coulomb energy for the TM 3d orbital. It is crucial to use DFT+U method and accurately account for the electron correlation in the calculations. By using linear response method, we are able to determine the Ueff value for all TM adatom. We find that the adsorption of TM atom not only efficiently modulates the electronic structures of GDY/GY system but also introduces excellent magnetic properties, such as spin-polarized half-semiconductor. Such modulation originates from the charge transfer between TM adatom and GDY/GY sheet as well as the electron redistribution of the TM intra-atomic s, p, and d orbitals. Our results indicate that the TM adsorbed GDY/GY are excellent candidates for spintronics.

Magnetic Properties of Single Transition-Metal Atom Absorbed Graphdiyne and Graphyne Sheet from DFT+U Calculations

The electronic and magnetic properties of single 3d transition-metal(TM) atom (V, Cr, Mn, Fe, Co, and Ni) adsorbed graphdiyne (GDY) and graphyne (GY) are systematically studied using first-principles calculations within the density functional framework. We find that the adsorption of TM atom not only efficiently modulates the electronic structures of GDY/GY system, but also introduces excellent magnetic properties, such as half-metal and spin-select half-semiconductor. Such modulation originates from the charge transfer between TM adatom and the GDY/GY sheet as well as the electron redistribution of the TM intra-atomic s, p, and d orbitals. Our results indicate that the TM adsorbed GDY/GY are excellent candidates for spintronics.

Magnetic Properties of Single Transition-Metal Atom Absorbed Graphdiyne and Graphyne Sheet

Raman scattering has been used to study the influence of manganese, an effective dopant to obtain ZnO diluted magnetic semiconductors, on the lattice dynamics of ZnO. It is found that Mn doping increases the lattice defects and induces two Raman vibration modes of 275 and 526cm−1. On the other hand, high temperature (TC higher than 350K) ferromagnetism is observed in Zn1−xMnxO (x⩽0.02) nanoparticles. It is found that the ferromagnetism of Zn1−xMnxO nanoparticles is strongly related to defects in ZnO.

Raman scattering and high temperature ferromagnetism of Mn-doped ZnO nanoparticles

Nanoring structure and optical properties of Ga8As8

Searching for two-dimensional (2D) materials with room-temperature magnetic order and high spin-polarization is essential for the development of next-generation nanospintronic devices. A new class of 2D magnetic materials with high Neel temperature, fully compensated antiferromagnetic order (zero magnetization) and completely spin-polarized semiconductivity is proposed for the first time. Based on the density functional theory calculations, we predict these properties for asymmetrically functionalized MXenes (Janus Cr2C) – Cr2CXX′ (X, X′ = H, F, Cl, Br, OH). The valence and conduction bands in these materials are made up of opposite spin channels and they can behave as bipolar magnetic semiconductors with zero magnetization. A Neel temperature as high as 400 K has been found for Cr2CFCl, Cr2CClBr, Cr2CHCl, Cr2CHF, and Cr2CFOH materials. Remarkably, the spin carrier orientation and induced transition from bipolar magnetic semiconductors to half-metal antiferromagnets can be easily controlled by electron or hole doping. The band gap of Janus MXenes can be effectively tuned by the selection of a pair of chemical elements/functional groups terminating the upper and the lower surfaces. The spin-polarized semiconductivity with zero magnetism is preserved when MXenes are put on the SiC(0001) support. The results presented herein open a new road towards the construction of 2D high-temperature spin-polarized materials with antiferromagnetism potentially suitable for spintronic applications.

Lizhong Sun

Papers

Magnetic Properties of Single Transition-Metal Atom Absorbed Graphdiyne and Graphyne Sheet from DFT+U Calculations

Magnetic Properties of Single Transition-Metal Atom Absorbed Graphdiyne and Graphyne Sheet

Raman scattering and high temperature ferromagnetism of Mn-doped ZnO nanoparticles

Nanoring structure and optical properties of Ga8As8

High temperature spin-polarized semiconductivity with zero magnetization in two-dimensional Janus MXenes